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13/06/201715/06/2017

June 13: Primary

June 13: Photographers: let’s not take colour for granted!

Today marks the birth of James Clerk Maxwell in 1831. To him we owe the first colour photograph, or at least to his assistant Thomas Sutton who was putting into practice the Scottish physicist’s theories on color and light to produce lantern slides which when aligned on a screen created a single colour image of a tartan ribbon.

On Friday 17 May 1861 Maxwell, who was then twenty-nine, gave a lecture On the Theory of Three Primary Colours to the Royal Institution of Great Britain, published in 1862 as “On the Theory of Three Primary Colours” in the Notices of the Proceedings at the Meetings of the Members of the Royal Institution of Great Britain with Abstracts of the Discourses delivered at the Evening Meetings, vol. 3 (1858-1862), p. 374 in which is the passage, so often quoted…

Three photographs of a coloured ribbon taken through the three coloured solutions respectively, were introduced into the camera, giving images representing the red, the green, and the blue parts separately, as they would be seen by each of Young’s three sets of nerves separately. When these were superposed, a coloured image was seen…

A young James Clerk Maxwell holding his colour wheel. Image: courtesy Trinity College Library, Cambridge University

It is interesting to untangle what this statement means. The speaker is a physicist who is not a photographer himself. Photography serves merely as a proof of his ideas. To read through the whole paper, as you can below, is to read one of the most lucid descriptions, the most insightful expression, of what colour is in human perception.

It is so perfect is it that it brings me to tears; it is logic, but also poetry.

Crucially it serves to show how our experience of colour as it comes to us through our senses is a specialised and particular effect of our organism…we see through our bodies!

What is remarkable, I should say visionary, revelatory, about this explanation is that he shows how the human colour experience is quite different to a scientific comprehension of colour, but that through it we are able to imagine what we cannot see.

This is an individual who wrote his first scientific paper at sixteen, who predicted what kind of material might form Saturn’s rings, then a phenomena that many had regarded as proof of divine perfection, that he proved mathematically to made of ‘brick-bats’, lumpy pieces, not of a continuous solid not of liquid, neither of which, he showed, could endure the planet’s gravity. He was proven to be correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Allegheny Observatory and Aristarkh Belopolsky of Pulkovo Observatory and by observations from Pioneer 11’s closest approach to Saturn in September 1979.

He was the one that envisaged the electromagnetic spectrum, who proved that light, magnetism and electricity were vibrations at different frequencies of the same form of energy, and proposed the existence of radio waves as a further part of this spectrum.

In his account for colour vision, and his proof of it [which is thus only incidentally the first colour photograph] he first refers to art, discussing the means by which painters achieve secondary colours and how, though their experiments, they had arrived at the three primaries red, yellow and blue, and the significance of this.

He then draws on the discoveries of predecessors; Isaac Newton (1642 – 1726), who showed, using prisms, that white light was composed of distinct colours; his own teacher, James David Forbes (1809 – 1868), who mixed colours using spinning tops which Maxwell employed to demonstrate that white light would result from a mixture of just red, green, and blue light; and also Thomas Young (1773 – 1829) who had hypothesised that colour perception depends on the presence in the retina of three kinds of nerve fibres.

Crucially, he points out the difference between the scientifically proven nature of colour and our human perception of it:

beams of light, passing through the transparent parts of the eye, fall on a sensitive membrane, and we become aware of various colours. We know that the colour we see depends on the nature of the light; but the opticians say there are an infinite number of kinds of light ; while the painters, and all who pay attention to what they see, tell us that they can account for all actual colours by supposing them mixtures of three primary colours.

Read his paper and you can see that the ‘filters’ he employed are ‘glass troughs containing respectively sulphocyanide of iron [producing intense red], chloride of copper [green], and ammoniated copper [blue]’, chemically the most pure examples of these colours that he could obtain.

Before moving on to the projected photograph, he validates Young’s theory by projecting light of the three chemically derived pure colours to form Young’s triangle (now known as Maxwell’s Triangle), the triadic colours and their intermixtures, on a screen.

When the time came to project with magic lanterns the three colour positive transparencies (lantern slides) he explained why not all of the colours of the ribbon were reproduced…

…if the red and green images had been as fully photographed as the blue, [it] would have been a truly-coloured image of the ribbon.

By this he means that the black and white emulsions used were unequally sensitive to the hues projected onto them through the chemically coloured liquid filters. He goes on to say that;

By finding photographic materials more sensitive to the less refrangible [able to be refracted] rays, the representation of the colours of objects might be greatly improved.

Such sensitivity was long in coming, the result of a quest by German chemist Hermann W. Vogel who extended sensitivity into the green, and later orange, by adding sensitising and compensating dyes to monochrome emulsion. However, his technique dd not achieve a fully panchromatic film until the early 1900s and it only became available commercially in 1906.

Maxwell’s assistant Thomas Sutton used one of the early iterations of the collodion process for his lantern slides, but had he used blue-sensitive daguerreotype to make transparencies (not possible with that technology), the result would have been rather disappointing as anything red, orange, yellow or green would appear as black.

I urge you to read the following transcription of Maxwell’s address to The Royal Institution which expresses discoveries about colour that all photographers should know:

On the Theory of Three Primary Colours.

The speaker commenced by shewing that our power of vision depends entirely on our being able to distinguish the intensity and quality of colours. The forms of visible objects are indicated to us only by differences in colour or brightness between them and surrounding objects. To classify and arrange these colours, to ascertain the physical conditions on which the differences of coloured rays depend, and to trace, as far as we are able, the physiological process by which these different rays excite in us various sensations of colour, we must avail ourselves of the united experience of painters, opticians, and physiologists. The speaker then proceeded to state the results obtained by these three classes of inquirers, to explain their apparent inconsistency by means of Young’s Theory of Primary Colours, and to describe the tests to which he had subjected that theory.

Painters have studied the relations of colours, in order to imitate them by means of pigments. As there are only a limited number of coloured substances adapted for painting, while the number of tints in nature is infinite, painters are obliged to produce the tints they require by mixing their pigments in proper proportions. This leads them to regard these tints as actually compounded of other colours, corresponding to the pure pigments in the mixture.

It is found, that by using three pigments only, we can produce all colours lying within certain limits of intensity and purity. For instance, if we take carmine (red), chrome yellow, and ultramarine (blue), we get by mixing the carmine and the chrome, all varieties of orange, passing through scarlet to crimson on the one side, and to yellow on the other; by mixing chrome and ultramarine we get all hues of green; and by mixing ultramarine with carmine, we get all hues of purple, from violet to mauve and crimson. Now these are all the strong colours that we ever see or can imagine: all others are like these, only less pure in tint.

Our three colours can be mixed so as to form a neutral grey; and if this grey be mixed with any of the hues produced by mixing two colours only, all the tints of that hue will be exhibited, from the pure colour to neutral grey. If we could assume that the colour of a mixture of different kinds of paint is a true mixture of the colours of the pigments, and in the same proportion, then an analysis of colour might be made with the same ease as a chemical analysis of a mixture of substances.

The colour of a mixture of pigments, however, is often very different from a true mixture of the colours of the pure pigments. It is found to depend on the size of the particles, a finely ground pigment producing more effect than one coarsely ground. It has also been shewn by Professor Helmholtz, that when light falls on a mixture of pigments, part of it is acted on by one pigment only, and part of it by another ; while a third portion is acted on by both pigments in succession before it is sent back to the eye. The two parts reflected directly from the pure pigments enter the eye together, and form a true mixture of colours ; but the third portion, which has suffered absorption from both pigments, is often so considerable as to give its own character to the resulting tint. This is the of the green tint produced by mixing most blue and yellow pigments.

In studying the mixture of colours, we must avoid these sources of error, either by mixing the rays of light themselves, or by combining the impressions of colours within the eye by the rotation of coloured papers on a disc.

The speaker then stated what the opticians had discovered about colour.
White light, according to Newton, consists of a great number of different kinds of coloured light which can be separated by a prism. Newton divided these into seven classes, but we now recognize many thousand distinct kinds of light in the spectrum, none of which can be shewn to be a compound of more elementary rays. If we accept the theory that light is an undulation, then, as there are undulations of every different period from the one end of the spectrum to the other, there are an infinite number of possible kinds of light, no one of which can be regarded as compounded of any others.

Physical optics does not lead us to any theory of three primary colours, but leaves us in possession of an infinite number of pure rays with an infinitely more infinite number of compound beams of light, each containing any proportions of any number of the pure rays.

These beams of light, passing through the transparent parts of the eye, fall on a sensitive membrane, and we become aware of various colours. We know that the colour we see depends on the nature of the light; but the opticians say there are an infinite number of kinds of light ; while the painters, and all who pay attention to what they see, tell us that they can account for all actual colours by supposing them mixtures of three primary colours.

The speaker then next drew attention to the physiological difficulties in accounting for the perception of colour. Some have supposed that the different kinds of light are distinguished by the time of their vibration. There are about 447 billions of vibrations of red light in a second; and 577 billions of vibrations of green light in the same time. It is certainly not by any mental process of which we are conscious that we distinguish between these infinitesimal portions of time, and it is difficult to conceive any mechanism by which the vibrations could be counted so that we should become conscious of the results, especially when many rays of different periods of vibration act on the same part of the eye at once.

Besides, all the evidence we have on the nature of nervous action goes to prove that whatever be the nature of the agent which excites a nerve, the sensation will differ only in being more or less acute. By acting on a nerve in various ways, we may produce the faintest sensation or the most violent pain ; but if the intensity of the sensation is the same, its quality must be the same.

Now, we may perceive by our eyes a faint red light which may be made stronger and stronger till our eyes are dazzled. We may then perform the same experiment with a green light or a blue light. We shall thus see that our sensation of colour may differ in other ways, besides in being stronger or fainter. The sensation of colour, therefore, cannot be due to one nerve only.

The speaker then proceeded to state the theory of Dr Thomas Young, as the only theory which completely reconciles these difficulties in accounting for the perception of colour.

Young supposes that the eye is provided with three distinct sets of nervous fibres, each set extending over the whole sensitive surface of the eye. Each of these three systems of nerves, when excited, gives us a different sensation. One of them, which gives us the sensation we call red, is excited most by the red rays, but also by the orange and yellow, and slightly by the violet; another is acted on by the green rays, but also by the orange and yellow and part of the blue; while the third is acted on by the blue and violet rays.

If we could excite one of these sets of nerves without acting on the others, we should have the pure sensation corresponding to that set of nerves. This would be truly a primary colour, whether the nerve were excited by pure or by compound light, or even by the action of pressure or disease.

If such experiments could be made, we should be able to see the primary colours separately, and to describe their appearance by reference to the scale of colours in the spectrum.

But we have no direct consciousness of the contrivances of our own bodies, and we never feel any sensation which is not infinitely complex, so that we can never know directly how many sensations are combined when we see a colour. Still less can we isolate one or more sensations by artificial means, so that in general when a ray enters the eye, though it should be one of the pure rays of the spectrum, it may excite more than one of the three sets of nerves, and thus produce a compound sensation.

The terms simple and compound, therefore, as applied to colour-sensation, have by no means the same meaning as they have when applied to a ray of light.

The speaker then stated some of the consequences of Young’s theory, and described the tests to which he had subjected it: —

1st. There are three primary colours.

2nd. Every colour is either a primary colour, or a mixture of primary colours.

3rd. Four colours may always be arranged in one of two ways. Either one of them is a mixture of the other three, or a mixture of two of them can be found, identical with a mixture of the other two.

4th. These results may be stated in the form of colour-equations, giving the numerical value of the amount of each colour entering into any mixture. By means of the Colour Top, such equations can be obtained for coloured papers, and they may be obtained with a degree of accuracy shewing that the colour-judgment of the eye may be rendered very perfect.

The speaker had tested in this way more than 100 different pigments and mixtures, and had found the results agree with the theory of three primaries in every case. He had also examined all the colours of the spectrum with the same result.

The experiments with pigments do not indicate what colours are to be considered as primary ; but experiments on the prismatic spectrum shew that all the colours of the spectrum, and therefore all the colours in nature, are equivalent to mixtures of three colours of the spectrum itself, namely, red, green (near the line E), and blue (near the line G). Yellow was found to be a mixture of red and green.

The speaker, assuming red, green, and blue as primary colours, then exhibited them on a screen by means of three magic lanterns, before which were placed glass troughs containing respectively sulphocyanide of iron, chloride of copper, and ammoniated copper.

A triangle was thus illuminated, so that the pure colours appeared at its angles, while the rest of the triangle contained the various mixtures of the colours as in Young’s triangle of colour.

The graduated intensity of the primary colours in different parts of the spectrum was exhibited by three coloured images, which, when superposed on the screen, gave an artificial representation of the spectrum.

Three photographs of a coloured ribbon taken through the three coloured solutions respectively, were introduced into the camera, giving images representing the red, the green, and the blue parts separately, as they would be seen by each of Young’s three sets of nerves separately. When these were superposed, a coloured image was seen, which, if the red and green images had been as fully photographed as the blue, would have been a truly-coloured image of the ribbon. By finding photographic materials more sensitive to the less refrangible rays, the representation of the colours of objects might be greatly improved.

The speaker then proceeded to exhibit mixtures of the colours of the pure spectrum. Light from the electric lamp was passed through a narrow slit, a lens and a prism, so as to throw a pure spectrum on a screen containing three moveable slits, through which three distinct portions of the spectrum were suffered to pass. These portions were concentrated by a lens on a screen at a distance, forming a large, uniformly coloured image of the prism.

When the whole spectrum was allowed to pass, this image was white, as in Newton’s experiment of combining the rays of the spectrum. When portions of the spectrum were allowed to pass through the moveable slits, the image was uniformly illuminated with a mixture of the corresponding colours. In order to see these colours separately, another lens was placed between the moveable slits and the screen. A magnified image of the lies was thus thrown on the screen, each slit shewing, by its colour and its breadth, the quality and quantity of the colour which it suffered to pass. Several colours were thus exhibited, first separately, and then in combination. Red and blue, for instance, produced purple ; red and green produced yellow ; blue and yellow produced a pale pink ; red, blue, and green produced white; and red and a bluish green near the line F produced a colour which appears very different to different eyes.

The speaker concluded by stating the peculiarities of colour-blind vision, and by shewing that the investigation into the theory of colour is truly a physiological inquiry, and that it requires the observations and testimony of persons of every kind in order to discover and explain the various peculiarities of vision.